You're assuming that they'd a few struggling bacteria - and that all of mankind wants to quit earth and spread as far and wide as possible. Both of which is basically bullshit. If anything - our population is heading towards its peak - which will follow with a sinking population. A majority of the developed world is greying - these are people who would have access to plenty of space and resources for their offspring - choosing not to have more. So I guess since you've taken on the role of the spokesperson for the "rest of mankind" - you might want to check in and see if humans are really that eager to go live on every rock in the solar system - or are fine where they are.
I don't have to run a poll. There's me, and folks like me. That's enough. Colonization never means 'most folks'; it always means only those willing to take risks and try something new. So your greying, senescent population can go ahead and stay at home without affecting anything regarding mankind's future.
I strongly suspect that if you took a poll, you would find that the real shortage will be a shortage of seats on rockets, not people to fill them. Even if people with a suitable exploratory attitude are one in a million, that still gives you thousands of volunteers.
Yes, even if they have to pay for it themselves. Someone interested in being an off-world colonist will have little use for earth assets; they could liquidate everything. Even if you peg it at tens of millions, you would still have more willing volunteers than rocket seats for the foreseeable future.
Though I am not sure why you assume that would necessarily be the scenario..
